1. Field of the Invention
The invention relates to determining the resistivity of geological formations through which a well provided with metal casing passes.
2. Description of the Related Prior Art
The importance of resistivity logging for oil prospecting is well known. It is known that the resistivity of a formation depends essentially on the fluid that it contains. A formation containing salt water, which is conductive, has resistivity that is much lower than a formation filled with hydrocarbons, and therefore resistivity measurements are of irreplaceable value for locating hydrocarbon deposits, Resistivity logging has been performed widely and for many years, in particular by means of apparatus having electrodes, but existing techniques have a field of application that is limited to wells that are not cased, or xe2x80x9copen holesxe2x80x9d as they are referred to in oil industry terminology. The presence in the well of metal casing, which has resistivity that is minute compared with typical values for geological formations (about 2xc3x9710xe2x88x927 ohmxc2x7m for steel casing compared with values in the range 1 ohmxc2x7m to 1000 ohmxc2x7m for a formation), represents a considerable barrier to sending electrical currents into the formations surrounding the casing. As a result, in particular, it is not possible to obtain resistivity measurements in wells that are in production, since such wells are provided with casing.
It would therefore be very advantageous to make it possible to measure resistivity in sections of cased wells. Such measurements, taken in a well in production at the level of the deposit, would make it possible to locate the water-hydrocarbon interfaces and thus to monitor how the positions of such interfaces vary over time, so as to monitor the behavior of the hydrocarbon reservoir and so as to optimize extraction therefrom. It would also be possible to obtain a resistivity measurement in a well (or a well section) for which no measurements were taken before the casing was put into place, in particular in order to supplement knowledge of the reservoir, and optionally to detect productive layers that were not located initially.
Proposals have been made on this subject in the literature. The basic measurement principle presented in Patent Document U.S. Pat. No. 2,459,196 consists in causing a current to flow along the casing under conditions such that current leaks out or is lost to the formation. Such loss is a function of the resistivity of the formation: the more the formation is conductive, the greater the current loss. By measuring current loss, it is possible to determine the resistivity of the formation. Current loss can be evaluated by measuring the voltage drop between electrodes placed at different depths in the well. Patent Document U.S. Pat. No. 2,729,784 describes a measurement method using two pairs of measurement electrodes a,b and b,c spaced apart along the casing, the electrodes a and c being in principle equidistant from the electrode b. Current electrodes are placed on either side of the measurement electrodes, so as to inject currents into the casing in opposite directions. A feedback loop servo-controls the injected current so as to put the is external measurement electrodes at the same potential, so as to cancel out the effect of the resistance of the casing varying in the sections (a,b) and (b,c) as defined by the measurement electrodes. A value for the leakage current at the level of the central electrode b is obtained by measuring the voltage drop across each of the pairs of electrodes a,b and b,c, and by taking the difference between the voltage drops, which difference is stated to be proportional to the leakage current.
French Patent Document 2 207 278 provides the use of three measurement electrodes spaced apart uniformly as in Patent Document U.S. Pat. No. 2,729,784 for measuring current leakage, and it describes a method in two steps: a first step for measuring the resistance of the casing section defined by the external measurement electrodes, during which step current is caused to flow along the casing so that there is no leakage into the formation; and a second step during which current can leak to the formation. For that purpose, a current injection system is provided that comprises one emission electrode and two return electrodes, a near one of the measurement electrodes being active during the first step, and the other measurement electrode being situated at the surface and being active during the second step.
Patent Document U.S. Pat. No. 4,796,186 describes a method in two steps of the same type as the method described in above-mentioned French Patent Document 2 207 278, and using the same electrode configuration. It provides a circuit for cancelling out the effect of resistance varying between the two sections of casing. That circuit comprises amplifiers connected to each pair of measurement electrodes so as to deliver respective voltage drops at their outputs. One of the amplifiers is a variable-gain amplifier, its gain being adjusted during the first step so as to cancel out the difference between the outputs or the amplifiers. Patent Document U.S. Pat. No. 4,820,989 describes all identical compensation technique.
An object of the invention is to provide this compensation function in a simple and effective manner.
The invention provides a method of surveying a geological formation through which a cased borehole passes, said method being characterized by the fact that:
in a first step, current is injected into the casing at a first point spaced apart longitudinally from said formation so as to cause current to Icak into said formation, and electrodes defining two consecutive sections of casing situated at the level of said formation are used to measure the respective potential drops along said sections;
in a second step, current is injected into the casing at a second point spaced apart longitudinally from the formation and situated on the opposite side thereof from said first point, so as to cause current to leak into the formation, and said electrodes are used to measure the potential drops along said sections;
the corresponding measurements of the two steps are combined to obtain the values corresponding to a circuit formed by the casing between the two injection points and essentially exempt from leakage into the formation, and
the current leakage that is indicative or the resistivity of the formation is determined on the basis of the measurements taken in the first step and in the second step, and of the values resulting from said combination.